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Ricin/ˈraɪsɪn/ is a highly toxic, naturally occurring lectin (a carbohydrate-binding protein) produced in the seeds of the castor oil plant Ricinus communis. A dose of purified ricin powder the size of a few grains of table salt can kill an adult human.[1] The median lethal dose (LD50) of ricin is around 22 micrograms per kilogram of body weight (1.78 mg for an average adult, around 1⁄228 of a standard aspirin tablet/0.4 g gross) in humans if exposure is from injection or inhalation.[2] Oral exposure to ricin is far less toxic, and an estimated lethal dose in humans is approximately 1 milligram per kilogram.[2]

Toxicity

Castor beans

Ricin is very poisonous if inhaled, injected, or ingested; it acts as a toxin by inhibiting protein synthesis.[3] It prevents cells from assembling various amino acids into proteins according to the messages it receives from messenger RNA in a process conducted by the cell's ribosome (the protein-making machinery)—that is, the most basic level of cell metabolism, essential to all living cells and thus to life itself. Ricin is resistant, but not impervious, to digestion by peptidases. By ingestion, the pathology of ricin is largely restricted to the gastrointestinal tract, where it may cause mucosal injuries; with appropriate treatment, most patients will make a full recovery.[4][5]

Because the symptoms are caused by failure to make protein, they emerge only after a variable delay from a few hours to a full day after exposure. An antidote has been developed by the UK military, although it has not yet been tested on humans.[6][7] Another antidote developed by the U.S. military has been shown to be safe and effective in lab mice injected with antibody-rich blood mixed with ricin, and has had some human testing.[8]Symptomatic and supportive treatments are available. Survivors often develop long-term organ damage. Ricin causes severe diarrhea, and victims can die of circulatory shock. Death typically occurs within 3–5 days of exposure.[9]

The seeds can be crushed in an oil press to extract castor oil. This leaves behind the spent crushed seeds, called variously the 'cake', 'oil cake', and 'press cake'. While the oil cake from coconut, peanuts, and sometimes cotton seeds can be used as either cattle feed and/or fertilizer, the toxic nature of castor precludes them from being used as feed.[10] Accidental ingestion of Ricinus communis cake to be used as fertilizer has been reported to be responsible for fatal ricin poisoning in animals.[3][11]

Deaths from ingesting castor plant seeds are rare, partly because of their indigestible capsule, and because the body can, although only with difficulty, digest ricin.[12] The pulp from eight beans is considered dangerous to an adult.[13] Rauber and Heard have written that close examination of early 20th century case reports indicates that public and professional perceptions of ricin toxicity "do not accurately reflect the capabilities of modern medical management".[14]

Overdose

Most acute poisoning episodes in humans are the result of oral ingestion of castor beans, 5–20 of which could prove fatal to an adult. However, there was one case of a 37 year old female ingesting 30 beans in the United States in 2013 who survived.[15] Victims often manifest nausea, diarrhea, tachycardia, hypotension, and seizures persisting for up to a week.[3] Blood, plasma, or urine ricin or ricinine concentrations may be measured to confirm diagnosis. The laboratory testing usually involves immunoassay or liquid chromatography-mass spectrometry.[16]

Biochemistry

Ricin is classified as a type 2 ribosome-inactivating protein (RIP). Whereas type 1 RIPs are composed of a single protein chain that possesses catalytic activity, type 2 RIPs, also known as holotoxins, are composed of two different protein chains that form a heterodimeric complex. Type 2 RIPs consist of an A chain that is functionally equivalent to a type 1 RIP, covalently connected by a single disulfide bond to a B chain that is catalytically inactive, but serves to mediate transport of the A-B protein complex from the cell surface, via vesicle carriers, to the lumen of the endoplasmic reticulum (ER). Both type 1 and type 2 RIPs are functionally active against ribosomes in vitro, however only type 2 RIPs display cytoxicity due to the lectin-like properties of the B chain. In order to display its ribosome-inactivating function, the ricin disulfide bond must be reductively cleaved.[17]

Structure

The quaternary structure of ricin is a globular, glycosylated heterodimer of approximately 60–65 kDa.[12] Ricin toxin A chain and ricin toxin B chain are of similar molecular weights, approximately 32 kDa and 34 kDa, respectively.

Ricin B chain (RTB) is a lectin composed of 262 amino acids that is able to bind terminal galactose residues on cell surfaces.[22] RTB forms a bilobal, barbell-like structure lacking alpha-helices or beta-sheets where individual lobes contain three subdomains. At least one of these three subdomains in each homologous lobe possesses a sugar-binding pocket that gives RTB its functional character.

Many plants such as barley have the A chain but not the B chain. People do not get sick from eating large amounts of such foods, as ricin A is of extremely low toxicity as long as the B chain is not present.

Entry into the Cytoplasm

Ricin B chain binds complex carbohydrates on the surface of eukaryotic cells containing either terminal N-acetylgalactosamine or beta-1,4-linked galactose residues. In addition, the mannose-type glycans of ricin are able to bind cells that express mannose receptors.[23] RTB has been shown to bind to the cell surface on the order of 106-108 ricin molecules per cell surface.[24]

The profuse binding of ricin to surface membranes allows internalization with all types of membrane invaginations. The holotoxin can be taken up by clathrin-coated pits, as well as by clathrin-independent pathways including caveolae and macropinocytosis.[25][26] Intracellular vesicles shuttle ricin to endosomes that are delivered to the Golgi apparatus. The active acidification of endosomes is thought to have little effect on the functional properties of ricin. Because ricin is stable over a wide pH range, degradation in endosomes or lysosomes offers little or no protection against ricin.[27] Ricin molecules are thought to follow retrograde transport via early endosomes, the trans-Golgi network, and the Golgi to enter the lumen of the endoplasmic reticulum (ER).[28]

For ricin to function cytotoxically, RTA must be reductively cleaved from RTB in order to release a steric block of the RTA active site. This process is catalysed by the protein PDI (protein disulphide isomerase) that resides in the lumen of the ER.[29][30] Free RTA in the ER lumen then partially unfolds and partially buries into the ER membrane, where it is thought to mimic a misfolded membrane-associated protein.[31] Roles for the ER chaperones GRP94,[32]EDEM[33] and BiP[34] have been proposed prior to the 'dislocation' of RTA from the ER lumen to the cytosol in a manner that utilizes components of the endoplasmic reticulum-associated protein degradation (ERAD) pathway. ERAD normally removes misfolded ER proteins to the cytosol for their destruction by cytosolic proteasomes. Dislocation of RTA requires ER membrane-integral E3 ubiquitin ligase complexes,[35] but RTA avoids the ubiquitination that usually occurs with ERAD substrates because of its low content of lysine residues, which are the usual attachment sites for ubiquitin.[36] Thus, RTA avoids the usual fate of dislocated proteins (destruction that is mediated by targeting ubiquitinylated proteins to the cytosolic proteasomes). In the mammalian cell cytosol, RTA then undergoes triage by the cytosolic molecular chaperones Hsc70 and Hsp90 and their co-chaperones, as well as by one subunit (RPT5) of the proteasome itself, that results in its folding to a catalytic conformation,[32][37] which de-purinates ribosomes, thus halting protein synthesis.

Ribosome inactivation

RTA has rRNA N-glycosylase activity that is responsible for the cleavage of a glycosidic bond within the large rRNA of the 60S subunit of eukaryotic ribosomes.[38] RTA specifically and irreversibly hydrolyses the N-glycosidic bond of the adenine residue at position 4324 (A4324) within the 28S rRNA, but leaves the phosphodiester backbone of the RNA intact.[39] The ricin targets A4324 that is contained in a highly conserved sequence of 12 nucleotides universally found in eukaryotic ribosomes. The sequence, 5’-AGUACGAGAGGA-3’, termed the sarcin-ricin loop, is important in binding elongation factors during protein synthesis.[40] The depurination event rapidly and completely inactivates the ribosome, resulting in toxicity from inhibited protein synthesis. A single RTA molecule in the cytosol is capable of depurinating approximately 1500 ribosomes per minute.

Depurination reaction

Within the active site of RTA, there exist several invariant amino acid residues involved in the depurination of ribosomal RNA.[27] Although the exact mechanism of the event is unknown, key amino acid residues identified include tyrosine at positions 80 and 123, glutamic acid at position 177, and arginine at position 180. In particular, Arg180 and Glu177 have been shown to be involved in the catalytic mechanism, and not substrate binding, with enzyme kinetic studies involving RTA mutants. The model proposed by Mozingo and Robertus,[21] based on X-ray structures, is as follows:

Therapeutic applications

Although no approved therapeutics are currently based on ricin, it does have the potential to be used in the treatment of tumors, as a so-called "magic bullet" to destroy targeted cells.[27] Because ricin is a protein, it can be linked to a monoclonal antibody to target malignant cells recognized by the antibody. The major problem with ricin is that its native internalization sequences are distributed throughout the protein. If any of these native internalization sequences are present in a therapeutic agent then the drug will be internalized by, and kill, untargeted non-tumorous cells as well as targeted malignant cells.

Modifying ricin may sufficiently lessen the likelihood that the ricin component of these immunotoxins will cause the wrong cells to internalize it, while still retaining its cell-killing activity when it is internalized by the targeted cells. However, bacterial toxins, such as diphtheria toxin, which is used in denileukin diftitox, an FDA-approved treatment for leukemia and lymphoma, have proven to be more practical. A promising approach for ricin is to use the non-toxic B subunit (a lectin) as a vehicle for delivering antigens into cells, thus greatly increasing their immunogenicity. Use of ricin as an adjuvant has potential implications for developing mucosalvaccines.

Chemical or biological warfare agent

The United States investigated ricin for its military potential during World War I.[43] At that time it was being considered for use either as a toxic dust or as a coating for bullets and shrapnel. The dust cloud concept could not be adequately developed, and the coated bullet/shrapnel concept would violate the Hague Convention of 1899 (adopted in U.S. law at 32 Stat. 1903), specifically Annex §2, Ch.1, Article 23, stating "... it is especially prohibited ... [t]o employ poison or poisoned arms".[44] World War I ended before the United States weaponized ricin.

The Soviet Union also possessed weaponized ricin. There were speculations that the KGB used it outside the Soviet bloc; however, this was never proven. In 1978, the Bulgarian dissident Georgi Markov was assassinated by Bulgarian secret police who surreptitiously "shot" him on a London street with a modified umbrella using compressed gas to fire a tiny pellet contaminated with ricin into his leg.[4][46] He died in a hospital a few days later; his body was passed to a special poison branch of the British Ministry of Defence (MOD) that discovered the pellet during an autopsy. The prime suspects were the Bulgarian secret police: Georgi Markov had defected from Bulgaria some years previously and had subsequently written books and made radio broadcasts that were highly critical of the Bulgarian communist regime. However, it was believed at the time that Bulgaria would not have been able to produce the pellet, and it was also believed that the KGB had supplied it. The KGB denied any involvement, although high-profile KGB defectors Oleg Kalugin and Oleg Gordievsky have since confirmed the KGB's involvement. Earlier, Soviet dissidentAleksandr Solzhenitsyn also suffered (but survived) ricin-like symptoms after an encounter in 1971 with KGB agents.[47]

Given ricin's extreme toxicity and utility as an agent of chemical/biological warfare, it is noteworthy that the production of the toxin is rather difficult to limit. The castor bean plant from which ricin is derived is a common ornamental and can be grown at home without any special care.

Ricin is several orders of magnitude less toxic than botulinum or tetanus toxin, but the latter are harder to come by. Compared to botulinum or anthrax as biological weapons or chemical weapons, the quantity of ricin required to achieve LD50 over a large geographic area is significantly more than an agent such as anthrax (tons of ricin vs. only kilogram quantities of anthrax).[49] Ricin is easy to produce, but is not as practical nor likely to cause as many casualties as other agents.[4] Ricin is inactivated (the protein changes structure and becomes less dangerous) much more readily than anthrax spores, which may remain lethal for decades. Jan van Aken, a German expert on biological weapons, explained in a report for The Sunshine Project that Al Qaeda's experiments with ricin suggest their inability to produce botulinum or anthrax.[50]

Developments

A biopharmaceutical company called Soligenix, Inc. has licensed an anti-ricin vaccine called RiVax™ from Vitetta et al. at UT Southwestern. The vaccine is safe and immunogenic in mice, rabbits, and humans. It has completed two successful clinical trials.[51]

^Augerson, William S.; Spektor, Dalia M.; United States Dept. of Defense, Office of the Secretary of Defense, National Defense Research Institute (U.S.) (2000). A Review of the Scientific Literature as it Pertains to Gulf War Illnesses. Rand Corporation, ISBN 978-0-8330-2680-4[page needed]

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Ricin is a legume lectin from the seeds of the castor bean plant,
Ricinus communis. The seeds are poisonous to
people, animals and insects and just one milligram of ricin can kill an adult.

Primary structure analysis has shown the presence of a similar domain in many carbohydrate-recognition proteins like plant and bacterial AB-toxins, glycosidases or proteases [PUBMED:9603958, PUBMED:7664090, PUBMED:8844840]. This domain, known as the ricin B lectin domain, can be present in one or more copies and has been shown in some instance to bind simple sugars, such as galactose or lactose.

The ricin B lectin domain is composed of three homologous subdomains of 40 amino acids (alpha, beta and gamma) and a linker peptide of around 15 residues (lambda). It has been proposed that the ricin B lectin domain arose by gene triplication from a primitive 40 residue galactoside-binding peptide [PUBMED:3561502, PUBMED:1881882]. The most characteristic, though not completely conserved, sequence feature is the presence of a Q-W pattern. Consequently, the ricin B lectin domain as also been refered as the (QxW)3 domain and the three homologous regions as the QxW repeats [PUBMED:7664090, PUBMED:8844840]. A disulphide bond is also conserved in some of the QxW repeats [PUBMED:7664090].

The 3D structure of the ricin B chain has shown that the three QxW repeats pack around a pseudo threefold axis that is stabilised by the lambda linker [PUBMED:3561502]. The ricin B lectin domain has no major segments of a helix or beta sheet but each of the QxW repeats contains an omega loop [PUBMED:1881882]. An idealized omega-loop is a compact, contiguous segment of polypeptide that traces a 'loop-shaped' path in three-dimensional space; the main chain resembles a Greek omega.

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Pfam Clan

This family is a member of clan Trefoil
(CL0066),
which has the following description:

This family corresponds to a large set of related beta-trefoil proteins [1]. The beta-trefoil is formed by six two-stranded hairpins [2]. Three of these form a barrel structure and the other three are in a triangular array that caps the barrel. The arrangement of the secondary structures gives the molecules a pseudo 3-fold axis.

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Interactive tree

For all of the domain matches in a full alignment, we count the
number that are found on all sequences in the alignment.
This total is shown in the purple box.

We also count the number of unique sequences on which each domain is
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Note that a domain may appear multiple times on the
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Finally, we group sequences from the same organism according to the
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We use the NCBI species tree to group organisms according to their
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Structures

For those sequences which have a structure in the
Protein DataBank, we
use the mapping between UniProt, PDB and Pfam coordinate
systems from the PDBe group, to allow us to map
Pfam domains onto UniProt sequences and three-dimensional protein
structures. The table below
shows the structures on which the RicinB_lectin_2
domain has been found. There are 51
instances of this domain found in the PDB. Note that there may be
multiple copies of the domain in a single PDB structure, since many
structures contain multiple copies of the same protein seqence.